The pattern and time course of reactive oxygen species (ROS) production were monitored histochemically in vivo in tobacco (Nicotiana tabacum L.) cv. Bel W3 and cv. Bel B leaves, comparing the hypersensitive (HR)-like reaction induced by the abiotic stressor ozone and the response to tobacco mosaic virus (TMV). The applied assays (nitro blue tetrazolium, NBT; 2',7'-dichlorofluorescin diacetate, DCFH-DA; 3,3'-diaminobenzidine, DAB) gave positive results, evidencing scattered ROS production in a spotted pattern. DCFH-DA and DAB predominantly evidenced localised, long-lasting hydrogen peroxide production in cells prone to die, producing very similar pictures in TMV infected leaves of both tobacco cultivars and in ozone-sensitive Bel W3. Nitro blue tetrazolium reaction pointed out that a transient oxidative burst involving superoxide anion is induced early by ozone also in the tolerant cv. Bel B, despite the low malondialdehyde level.